Francis-type runners can be fitted to various kinds of hydraulic machines such as turbines, pumps or turbine-pumps. They comprise blades distributed about a central rotation shaft and between them defining ducts for the flow of water. The geometry of the blades of these runners is defined so that the flow of the water imparts a rotational torque to the runner when it is being used in turbine mode.
A constant preoccupation with Francis-type runners for turbines is that of controlling the flow passing through these runners and of optimizing it both for the point of best efficiency and under full and part mode conditions.
The use of roughly triangular vanes to improve the operation of a Francis-type runner is known, for example from U.S. Pat. No. 4,017,211, the vanes being fixed to the bottom part of the crown at the outlet end of the blades to divert the outlet flow from the blades to eliminate any swirling flow. This runner structure entails the addition of vanes at the crown and these may create a certain discontinuity in the flow, which make the structure of the runner more complicated, and are additional elements that have to be manufactured and assembled separately.
Another solution known from EP-A-0 930 432 is to modify all the blades of a Francis-type runner for a turbine by adding a point of inflection to their trailing edges, near the crown. This blade geometry makes it possible to relieve mechanical stresses on the runner but leads to a discontinuity in the flow downstream of the runner. Furthermore, this blade structure may present problems with fitting all the blades on the crown of the runner because the points of attachment of the trailing edges of all the blades to the crown are concentrated into a small area.
It is these disadvantages that the invention more particularly seeks to overcome by proposing a new Francis-type runner for a turbine, the flow of which is optimized, both at the point of greatest efficiency and under full and part load conditions, without the addition of additional components and without giving rise to particular problems of fitting into a given space.